The present invention relates generally to methods of processing semiconductor waters, and more particularly to a method of chemical mechanical polishing (CMP) for semiconductor wafers.
In a conventional chemical mechanical polishing apparatus, a semiconductor wafer is adhered to a polishing block (broadly, “mounting member”) of the apparatus. The polishing block is typically held on a hub of the apparatus by vacuum pressure. The wafer is rotated on the hub and forced downward by the hub against a rotating polishing pad which is mounted on a turntable. Heat and pressure created during the polishing process may deform the polishing block and/or the pad into a slightly concave or convex shape during polishing. Additionally, the pad shape and the pad surface characteristics may change after processing a number of wafers. The charge in shape of the block and the change in shape or surface of the pad can cause the downward pressure exerted on the wafer by the hub to be non-uniform over the wafer surface and thereby affect flatness and parallelism of the surfaces of the polished wafer. It is known in the art that adjusting the vacuum pressure on the polishing block can deform the block and the wafer in ways which tend to counteract the deformation of the block or pad caused by other factors and thereby maintain the flatness and parallelism of the wafer. However, the conventional method of adjusting vacuum pressure relies on the skill and experience of the operator to determine the amount of adjustment and is not dictated by a predetermined procedure. Therefore, the results of this manipulation are not consistent.
The polishing pad of the chemical mechanical polishing apparatus is relatively expensive and must be of high quality to produce wafers of satisfactory flatness and roughness. During polishing, the wafer and pad are rotating at different velocities and their axes are not concentric. As a result, portions of the pad are contacted more often than ocher portions by the wafer material, and wearing (or “aging”) of the pad becomes uneven. Such uneven pad wear affects the flatness and parallelism of the polished wafers so that the pad must be replaced prematurely in order to maintain wafer flatness and parallelism.